Microwave oscillator having frequency and amplitude stabilization means



llg. 9, 1966 5, B, MARSH ET Al.. 3,265,985

MICROWAVE OSCILLATOR HAVING FREQUENCY AND AMPLITUDB l STABILIZATION MEANS Flled Deo. 19, 1961 2 Sheets-Sheet l tors.

MrcRowAvn osCiLLA'ron HAVING FREQUENCY AND AMPLHUDE srauiLizA'rroN MEANS .I Stanley Bruce Marsh and Allan Stanley Wiltshire, Mal- Claims priority, application Great Britain, Dec. 30, 1966,

This invention relates to the stabilisation of electronic oscillators and Ahas vreference to oscillators from which it is desired to'obtain' a high power oscillation, at a controlled frequency and with low `ncrise modulation side bandsin the range of frequencies known inthe radar field as the Doppler band, e.g. 1-;60.kc./s. e

' Generally oscillators used vin the transmitters of CW radar systems comprise a klystron oscillator followed by a klystronA amplifier. Conventional stabilisation techniques are not very effective when applied 'to such transf mitters owing tothe stringent low-noise .requirements which must vbe met to avoid false Doppler signals in the radar system due to signals from ground clutter. For instance a 2 kw. CWv automatic tracking radar transmitter may consist of a two-resonator klystron oscillator followed by two two-resonator klystron amplifiers in cascade; power v levelsare set yby power dividers to determine the input to `the intermediate amplifier and optinuim` drive .to the final amplifier. lThetransmitter isV used. ina CW radarsystem in whichtarget selection is'made by measurement of Dop' ple'r shift in reflected signals, the Doppler frequencies lying in the range. 1 6() kc./s.4 -Inorder that reflections from ground clutter shall not'give '.rise'tov false signals in the eg.' 1 -60 kc./ s. Random frequency modulation is similarly limited in sideband power. l

Frequency and amplitude noise modulation which occurs in-such a transmitter has been identified with fluctual tions ofthe ,current in the electrn beams rin the oscilla tor 'and amplfier'v'alves and is random in character. It is `clearly necessary to reduce this noise modulation and close attention' in the manufacture lof the valves to avoidance of dust within the 'vacuum envelope results in a considerable improvement; but further reduction still appearsmost`desir'able if the vfull vpotentialities of the transmitters are to be realised. Y

It is an objectof the present invention thereforeto provide an 'improvedlvmeans-'for stabilising electronic oscilla- It has beenV realised that therandorn frequency modula tion noise arises-mainly in the oscillator of a CW transtesPatflt OH umplcr' u two-orl`our vcavity klyslron whose output nmpliludc is controlled by a ferrite or semiconductor :impliluilc modulator device; frequency control of the oscillator is by control of the E.H.T. voltage applied to it-lhis iutroduces` negligible amplitude variation-zuid control ot the amplitude modulator is by means of a current passing through the coil ofa ferrite modulator device or a voltage applied to a terminal of a semiconductor-modulator Vdevice.

The meansfor producing error signals is conveniently a frequency discriminator of the bridge type described in a paper read before the Institution of Electrical ,Engineers Conference on Microwave Measurements held in London September 6-8, 1961: T he Use of a Microwave Discriminator in the Measurement of Noise Modulation, and

the subject of U.S. Patent No. 3,079,563, Feb. 26, 1963,

the: oscillator to control the mean ,oscillation frequency;

f this` also maintains the discriminator bridge in balance.'

` An isolator may be provided between the basic cs cilla-4 tor and the amplifier to reduce pulling of the oscillatorand consequent frequency modulation noise incurred by variation ofthe amplifier beam current.-

" In order to makethe invention clearer an example of a stabilised oscillator will now be described with 'reference to the accompanying drawings in which:

FIGURE l is a circuit diagram of a' stabilised oscillator incorporated in a CW radar transmitter;

FIGURE'Z is a lcircuit diagram of an alternative part of the circuit shown in FIGURE l; and

FIGURE 3 is a circuit diagram of another alternative l part of the circuit shown in FIGURE 1.

In FIGURE l an oscillator 1, conveniently of the ytwocavity klystron type,- the frequencyv of which is controlled by varying the E.H.T. voltage on its reflector yis connected via a ferrite isolator 2 Vto a two-kly'stron amplifier 3 consisting of two stages 3A,l 3B; level adjusting impedances 3AA, 3BE are provided at the input to each stage. y

An vE.H.T. stabiliser'ci'rcuit 5,y comprising a voltage source substantially independent of'powe'r and load variation, ripple, etc., stabilises the4 E.H.T. of the oscillator 1,

and a voltage modulator 6 is provided in series with thes'tabilised E.H.T. supply t'o the oscillator 1.

A small fraction of the output of the second amplifier stage 3B is passed throughV a ferrite modulator 4 by means of a directional coupler 18, a portion of the output of the modulator 4 being fed back intoa main output waveguide 7 by means of a directional coupler 19 through a phase shifter 20. In a typical example the couplers 18 and 19 each have a 'coupling ratioof 23 db. Thelferrite 'modulator may be of conventional construction; an example of a suitable'modulator is described by A. L. Morris in an article in Journal Brit. I-.R.E., vol. 19, No 2, February 1959,page117.

mitter'and the random 'amplitude modulation noise arises feeding its output' to the amplifier and means vfor produ-cing error signals indicating'frequency error and ampli- 'rude-error in the output'oscillation of the stabilisedy oscillator) vand for applying them tothe basicoscillator andthe amplifier respectively to vcontrol theoutput frequency and amplitude.-

`The'controlled output of the amplifier 3 is fed via the output waveguide 7 to a utilisation means 8, in the present example 'transmitter aerial. g n

A microwave suppressed carrier frequency discriminator 9 of the bridge type described in U.S. Patent No.

'3,079,563,*Feb. 26, 1963, Stanley Bruce Marsh and John *conveniently the oscillator is twofclyity "kiystmn, the

Dickens Clare, is coupled at its input directional coupler 10 to the transmitter output waveguide 7. An output 12 'provides-fa signalv indicating frequency variation or errorl ofthe transmitter output in the waveguide 7 and an output 11 a signalindicating amplitude variation or error of the transmitter output.

We note that the basic internal arrangement. of the disc'riminator 9 has been described and discussed arlength in theMar'sh and Wiltshire paper `to' the I.E.E. (loc. cit.)v and lU.S. Patent No. 3,079,563, Feb. 26, 19673, Stanley asf-Gansta 3 Bruce Marsh and John Dickens Clare, in the interests of brevity, will not he referred to in the Same detailed tuanner here, eXcept to mention that the measurements of FM noise modulation and AM noise modulation, used in the paper 'and pat-ent, respectively denote similar meas- A D.C. amplifier 17 also is connected to the frequency error output 12 and feeds a D CL control signal to the stabiliser circuit 5'.- In operation theoutput of the oscillator 1 is amplified by the amplifier 3 and fed as a transmitter output signal via the waveguide 7 to the aerial 8.

A sample of the transmitter outputsignal passes into the frequency discriminator 9 by virtue of the directional coupler y10 and signals are obtained at the outputs 11. and 12` of the -discrirninator 9 which indicate amplitude and frequency lerror in' the transmitter output signal.

The frequency error signal (output'12') passesthrough the'amplifier 14 to the modulator 6 and a control loop is thereby completed for controlling the frequency of the oscillator 1. amplifier 14, over the range covered bythe amplifier.

The amplitude error signal (output 11) 'passes through the amplifier 16 to the ferrite modulator 4 which controls the feeding of a 'small quantity of energy from the waveguide 7 via the coupler 18 back into the waveguide 7 viathe coupler 19. The control loop so formed is'brought into adjustment 'by means of the phase adjuster 20 and is' effective-in controlling the amplitude of the output of the amplifier 3 Again vthis controiacts, by virtue of the amplifier 16, over the range of Doppler frequencies covered by 'the' amplifier.

Thusthe transmitter is stabili'sed againstvvariations of frequency and amplitude occurring at frequencies within the Doppler range.

The frequency error signal fed to the E.H.T. stab-ilising circuit 5 via the D.C. amplifier 17 controls the mean fre- `of Doppler frequencies quency ofthe oscillator l; also at the same time, because theerror signal output -of the D.C. amplifier 17 is 4proportional to any unbalance which exists in the bridge of the frequency discriminator 9 the balance of the bridge is automatically maintained.

Although la frequency discriminator has 'been described for producing amplitude error signals a'vreceive'r of the superhcterodyne type with anenvelope detector at intermediate' frequency may be used'instead. This would give 'a uniform sensitivity over the Doppler frequency band 1-60 kc./s. inthe present example) It should be noted, however, that'the'use of the fre- I quency'discrirninator har/ing an R.F. discriminator and F phase sensitive detectors (the sign detectorsof the discriminator 9) with-the reference carrierin quadrature with the reference carrier used for deriving the frequency error signal, results-in a sensitivity which' improves as the modulating frequency increases at the rate of 6 db perv octave; asl long as time delay inthe discriminator is substantially less than a quarter period of the highest noise modulation frequency (ie. of the highest frequencyof thev frequencyerror signal). This sensitivity characteristie-is most advantageous in a CW radar transmitter-as it is of the form which reduces the `effects of long range clutter.

If' we consider the more detailed design of the example y of a stabilised oscillator we seegthat .it is not strictly essential that the D.C.and Doppler frequency amplifiers 17,

vThe control is effective, by virtue of the .14 in the frequency error loop are separate amplifiers; but in practice it is preferable that the) should hc, so that with finite and reactive output impedance ofitht` klystrou power supply. high stable gain may be maintained. The gain of the overall control loop in which the frequency error signal is effective is limited mainly by time delay in the discriminator itself. The low frequency response of the loop is limited by the output impedance of the power supply.

The maximum gain of the v.amplitude error loop incorporating a discriminntor in the circuit shown is similarly limited by time delay in the discriminator.

As an alternative tothe ferrite modulator 4 of the circuit-described'with reference to FIGURE' l a semiconductor modulator may be employed 'as shown in FfGURES 2 and 4. vin FIGURE 2a a semiconductor' diode 21 of 'the' so-called vat-actor type or of the type used in microwave mixer circuits is locatedin the waveguide of an output arm 22 of a directional coupler 23 the other output arm 24 being terminated by a dummy load 25. The input arms of the directional coupler 23 are connectedto the output arms of .a second directional coupler 26, a variable phase-shifter 27 being provided conveniently in one of the connections between the input .arms of the first coupler 23 and the output arms of the second coupler 26,.

T he input arms of the .second coupler are connected in the stabilised oscillator circuit described to those arms of the couplers 1S and 19 which are connectedA to the ferrite modulator d and the phase .shifter 20 respectively. The modulator 4 and the phase-shifter 20 are, of course', not then included and the output 28 of the Doppler amplifier 16 is connected to one terminal of the varactor or mixer type diode 21, the other of which is connected to a suitable potential source. In FIGURE 3, which is another-alternative to the ferrite modulator, a 2t) db directional coupler 29 is substituted for the two couplers 18 ajnd 19 of the circuit described and its parallel input and-output arms arelconnected to the input arms of a 17 dbdirectional coupler 3f) the output arms of which are .connected correspondingly to a varactor or mixer-type diode Ziand a dummy load 31., The diode, as in the previous example, is connected to the output 28 of Doppler amplifier 16, the ferrite modulator 4 andthe phase shifter v 20 not then of course being used.

In general, improvements in stabilisation obtained dpend upon the sensitivity ofthe discriminator, which in turn -is determined by themagnitude ofthe signal which can be applied to it.. Thus improvement of the balance of the discriminator. bridge improves the sensitivity and the improved arrangement described in U.S.l Patent No. 3,197,707, issued lon July 27, 1965, to S. Wiltshire and S. A. Drage maybe employed to advantage here.

It will be appreciated that, in the example described, lthe amplifier 3 forms part ofthe `frequency error control loop. Thisis not, of course, strictly necessary as the frequency error information can be obtained from the output of the oscillator 1; however, the frequency modulation noise in the amplifier 3, although small, is not small relative to that which occurs in the frequency controlled oscillator and it is preferred therefore that the amplifier form part of the control loop.

What we claim is: y 1. A stabilised microwaveoscillation generator including an oscillator circuit operative at microwave frequencies and subject to amplitude and frequency errors in its output, comprising:

' control means connected to said first output for controlling the microwave output amplitude of said oscillator circuit, and

control means connected to said second output for controlling the microwave frequency of oscillation of said oscillator circuit,

said oscillator circuit including amplifying means for controlling the output amplitude of said oscillator circuit,

said amplifying means including an output signal channel comprising a main waveguide and a .branch waveguide,

the branch waveguide having an input and an output means each connected to the main waveguide,

said .amplitude controlling means including a modulator in said branch waveguide connected to said first output and phase adjusting means in said branch waveguide.

Z. A stabilised oscillation generator as claimed in claim 1 wherein the said oscillator circuit includes a klystron and the said means for controlling the frequency of oscillation thereof includes means for controlling the reflector voltage of said klystron.

3. A stabilised oscillation generator as claimed in claim 1 wherein said oscillator circuit includes a klystron; and wherein said means for controlling the frequency of oscillation of said oscillator circuit comprises a stabilised voltage source and associated voltage modulator means for supplying reflector voltage to said klystron, means for operatively connecting a direct current component of said second output to the stabilised voltage source, and means for connecting an alternating current component of said second output to said voltage modulating means.

4. A -stabilised microwave oscillator generator as claimed in claim 1 wherein said modulator in said branch waveguide is a ferrite waveguide modulator.

5. A stabilised microwave oscillator generator as claimed in claim 1 wherein said modulator in said branch Vwaveguide is a semiconductor modulator.

6. A stabilised oscillation generator as claimed in claim 5 wherein the said oscillator circuit includes a klystron and the said means for controlling the frequency of oscillation thereof includes means for controlling the reflector voltage of said klystron.

7. A stabilised oscillation generator .as claimed in claim 5 wherein the semiconductor modulator comprises a diode of the varactor type.

8. A stabilised oscillation generator as claimed in claim 5 wherein the semiconductor modulator comprises a diode of the microwave mixer type.

9. A stabilised oscillation generator as claimed in claim 5 wherein said oscillator circuit includes a klystron; and wherein said means for controlling the frequency of oscillation of said oscillator circuit comprises a stabilised voltage source and associated voltage modulator means for supplying reflector voltage to said klystron, means for operatively connecting a direct current component of said second output to the stabilised voltage source, and means for connecting an alternating current component of said second output to said voltage modulating means.

10. A stabilised microwave oscillation generator including an oscillator circuit operative at microwave fre- 6 quencies and subject to amplitude and frequency errors in its output, comprising:

a microwave discriminator means connected to the output of said oscillator circuit and operative on said output for generating error signals in the form of a rst output indicative of said amplitude error in the output of said oscillator circuit and a second output indicative of said frequency error in said oscillator output,

control means connected to said first output for controlling the microwave output amplitude of said oscillator circuit, and

control means connected to said second output for controlling the microwave frequency of oscillation of said oscillator circuit,

said oscillator circuit including amplifying means for controlling the amplitude of said oscillator circuit,

said oscillator circuit including a klystron, and

said control means for controlling the microwave frequency of oscillation includes means for controlling the reector voltage of said klystron.

11. A stabilised microwave oscillation generator including an oscillator circuit operative at microwave frequencies and subject to amplitude and frequency errors in its output, comprising:

a microwave discriminator means connected to the output of said oscillator circuit and operative on said output for generating error signals in the form of a rst output indicative of said amplitude error in the output of said oscillator circuit and a second output indicative of said frequency error in said oscillator output,

control means connected to said rst output for controlling the microwave output amplitude of said oscillator circuit, and

control means connected to said second output for controlling the microwave frequency of oscillation of said oscillator circuit,

said oscillator circuit including amplifying means for controlling the output amplitude of said oscillator circuit,

said oscillator circuit including a klystron, and

said means for controlling the microwave frequency of oscillation of said oscillator circuit comprises a stabilised voltage sou-rce and associated voltage modulator means for supplying reflector voltage to said klystron,

means for operatively connecting a direct current component of said second output to the stabilised voltage source, and

means for connecting an alternating current component of said second output to said voltage modulating means.

References Cited by the Examiner UNITED STATES PATENTS 2,172,456 9/1939 Schock 331-15 2,713,122 7/1955 Henley 331-6 2,931,900 4/1960 Goodman 331-15 ROY LAKE, Primary Examiner. 

1. A STABILISED MICROWAVE OSCILLATION GENERATOR INCLUDING AN OSCILLATOR CIRCUIT OPERATIVE AT MICROWAVE FREQUENCIES AND SUBJECT TO AMPLITUDE AND FREQUENCY ERRORS IN ITS OUTPUT, COMPRISING: A MICROWAVE DISCRIMINATOR MEANS CONNECTED TO THE OUTPUT OF SAID OSCILLATOR CIRCUIT AND OPERATIVE ON SAID OUTPUT FOR GENERATING ERROR SIGNALS IN THE FORM OF A FIRST OUTPUT INDICATIVE OF SAID AMPLITUDE ERROR IN THE OUTPUT OF SAID OSCILLATOR CIRCUIT AND A SECOND OUTPUT INDICATIVE OF SAID FREQUENCY ERROR IN SAID OSCILLATOR OUTPUT, CONTROL MEANS CONNECTED TO SAID FIRST OUTPUT FOR CONTROLLING THE MICROWAVE OUTPUT AMPLITUDE OF SAID OSCILLATOR CIRCUIT, AND CONTROL MEANS CONNECTED TO SAID SECOND OUTPUT FOR CONTROLLING THE MICROWAVE FREQUENCY OF OSCILLATION OF SAID OSCILLATOR CIRCUIT, SAID OSCILLATOR CIRCUIT INCLUDING AMPLIFYING MEANS FOR CONTROLLING THE OUTPUT AMPLITUDE OF SAID OSCILLATOR CIRCUIT, SAID AMPLIFYING MEANS INCLUDING AN OUTPUT SIGNAL CHANNEL COMPRISING A MAIN WAVEGUIDE AND A BRANCH WAVEGUIDE, THE BRANCH WAVEGUIDE HAVING AN INPUT AND AN OUTPUT MEANS EACH CONNECTED TO THE MAIN WAVEGUIDE SAID AMPLITUDE CONTROLLING MEANS INCLUDING A MODULATOR IN SAID BRANCH WAVEGUIDE CONNECTED TO SAID FIRST OUTPUT AND PHASE ADJUSTING MEANS IN SAID BRANCH WAVEGUIDE. 